The four course radio range (also known as the "low frequency radio range," or "LFR"), is a now-defunct navigation system which was replaced by the VOR. The Civil Aeronautics Administration commissioned the first VOR in 1947, and the final LFR, in Northway Alaska, was decommissioned in 1974. The low frequency radio range system had originally been commissioned by the U.S. Commerce Department on 30 June 1928.

Four Course Radio Range Station at Northway, Alaska

The radio range station emitted a directional signal, forming four beacons aligned with respect to the compass, each defining a course. Pilots listened to a radio receiver and followed these radio beams from station to station along the route. The four-course radio range system was phased out beginning in 1950, after reaching a maximum deployment of 378 stations.

Daggett Radio Range depicted on US Airway
Map Los Angeles (Upper I-11), June 1932.
(Click on image to view much larger (1.92 meg) version which will open in a new window)

The Daggett radio range, shown above, was located 14 miles SW of the current Daggett (DAG/KDAG) VOR. This installation was commissioned in 1932 as transmitter number 13 and decommissioned in 1972, at which time the equipment was donated to the Smithsonian.

Here's how it works:

The radio range station transmits three different signals:

1. A station identification signal;
2. A morse code "N"; and,
3. A morse code "A."

The morse code "A" signal forms an hourglass shape as does the "N" signal, and the "A" and "N" signals are at right angles to each other (see the diagram below).

When an aircraft is in one of the "clear quadrants" (the red or green areas), a clear morse code signal is received. In the green areas the crew hears a morse code "N" in addition to the station identification signal. In the red areas, the crew hears a morse code "A" plus the station identification signal.

In our diagram, aircraft 1 is in an "N" quadrant and hears a morse code "N" along with the station ID.

As the aircraft approaches the range leg (the dark gray area labeled "on course"), the "N" signal decreases in intensity and a background hum increases. This hum is a result of the overlap of the "N" signal from the quadrant the aircraft is leaving and "A" signal from the quadrant the aircraft is approaching. In our diagram, aircraft 2 is hearing a decreasing "N" signal and a growing hum from the overlap of the "A" and "N" signals.

As the aircraft crosses the range leg (aircraft 3) the hum caused by the signal overlap decreases and the "A" signal from the quadrant being entered grows in intensity. In the "Bi-Signal" areas the crew hears both "A" and "N" signals.

An aircraft on the range leg (aircraft 4) hears only the hum of the overlapping signals.

When an aircraft is directly over the station the crew hears nothing at all. The "cone of silence" is cone-shaped and the diameter of the cone increases with aircraft altitude. Aircraft entering the cone will hear the station signal increase rapidly and then suddenly disappear. After station passage, the signal suddenly reappears and decreases in strength as the aircraft moves away from the station.

On a final note... the star-shaped indicators on the US Airway Map (i.e. 9, 10A, etc), are Airway Light Beacons--coded light beacons which can be identified from their sequenced flashes. For instance, beacon 10A produces two long flashes, whereas beacon nine produces two longs and a short. The arrows shown with the stars indicate the course to be flown. Airway Light Beacons predate all radio aids to navigation.

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See Also:

• Ultra High Frequency Radio Range
• Very High Frequency Radio Range
• Z Marker
• Manual for Flight Simulator Radio Range add-on